Boeing alleges the unfair subsidy stems from when Northern Ireland's power-sharing administration and the UK Government pledged to invest almost £135m to help set up a C-Series manufacturing site in Belfast.

An independent USA trade body still has to rule on whether Boeing suffered any harm from Bombardier's tactics when the Canadian company won a big sale of 75 CSeries jets previous year to Delta Air Lines Inc. and unsuccessfully bid to sell planes to United Continental Holdings Inc.

The duty would apply to the cost of CSeries planes imported to the United States, effectively keeping it out of the market.

The U.S. Commerce Department was expected to announce preliminary anti-dumping duties yesterday, but last night said it was holding off until today.

The duties, which would affect an order for 75 planes by Delta Air Lines, would not take effect unless approved by the U.S. International Trade Commission early next year.

The total duty was well above the 80 percent Boeing sought in its complaint.

Components of the C-Series jet are manufactured at a purpose-built factory in east Belfast and many other local firms are involved in the supply chain. The controversy also could hang over his trip to Washington next week, where he is scheduled to discuss trade with President Donald Trump just as negotiators hold the fourth round of talks to amend the North American Free Trade Agreement.

"These duties are the outcome of a conscious decision by Bombardier to violate trade law and dump their CSeries aircraft to secure a sale", Chicago-based Boeing said in a statement.

The U.S. imposed duties on Bombardier Inc.'s showcase commercial jet for the second time in as many weeks, upholding Boeing Co.'s case that its Canadian competitor sold planes in the U.S.at less than fair value.

"Bombardier always has the option of coming into full compliance with trade laws", Boeing added.

USA commerce secretary Wilbur Ross said: "The United States is committed to free, fair and reciprocal trade with Canada, but this is not our idea of a properly functioning trading relationship".

"We will.do everything in our power to stand up for American companies and their workers", Ross said in a statement. The plane supports an estimated 22,700 jobs and Bombardier's aerospace division spent $2.14 billion in the United States a year ago, according to the company and documents seen by Reuters.

The US Department of Commerce has again ruled against aerospace firm Bombardier in its dispute with rival Boeing. More than half of the purchased content of each CSeries aircraft comes from US suppliers, Bombardier has said.